Method for predicting and avoiding a bad bond when utilizing fiber push connect laser bonding

ABSTRACT

The disclosure describes a method for predicting and avoiding bad bonds or connections when performing electrical connection of two electrical conductors by using a laser light beam attached to an optical fiber system which directs the light to the spot to be bonded. The method provides for rapid detection of damaged optical fibers before bad bonds or connections occur. Disclosed is a method for predicting and avoiding bad bonds or connections when performing solderless electrical connection of two contact elements by using a laser light beam attached to a optical fiber system which directs the light to the spot to be bonded. The method of the present invention performs optical fiber push laser bonding operations on electric conductor leads includes providing an optical fiber push laser bonding system having an optical fiber for directing a laser beam, positioning first and second electrical leads in a bonding position, holding the first and second electrical leads in contact at a bond surface with an optical fiber, bonding the first and second electrical leads at the bond surface by directing the laser beam through the optical fiber, repeating said positioning, holding and bonding steps for a plurality of bonds, interrupting the aforesaid laser bonding operations in order to examine the condition of the fiber; wherein the following procedures occur during said interrupting directing the laser beam through the optical fiber, capturing the spatial energy distribution of the laser beam exiting the optical fiber and analyzing the spatial energy distribution of the laser beam to determine condition of the optical fiber in order to determine the need for corrective action.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation in part of co-pending U.S.patent application Ser. No. 08/843,492, filed Apr. 16, 1997, entitled,“Solderless Connection of Electrical Contacts Utilizing CombinationLaser and Fiber Optic Push Connect System,” which is a continuation ofU.S. patent application Ser. No. 08/558,567, filed Oct. 31, 1995,entitled, “Solderless Connection of Electrical Contacts UtilizingCombination Laser and Fiber Optic Push Connect System” now abandoned andis related to U.S. patent application Ser. No. 08\705,928, filed Aug.30, 1996, entitled “Laser Welded Inkjet Printhead Assembly Utilizing aCombination Laser and Fiber Optic Push Connect System.” The foregoingcommonly assigned U.S. patent applications are herein incorporated byreference.

FIELD OF THE INVENTION

[0002] The present invention generally relates to the electricalconnection of two elements and, more particularly, to the solderlessconnection of two elements using an optical fiber that holds theelectrical elements in contact while directing a laser emission to thelocation to be bonded.

BACKGROUND OF THE INVENTION

[0003] Thermal inkjet print cartridges operate by rapidly heating asmall volume of ink to cause the ink to vaporize and be ejected throughone of a plurality of orifices so as to print a dot of ink on arecording medium, such as a sheet of paper. The properly sequencedejection of ink from each orifice causes characters or other images tobe printed upon the paper as the printhead is moved relative to thepaper.

[0004] An inkjet printhead generally includes: (1) ink channels tosupply ink from an ink reservoir to each vaporization chamber proximateto an orifice; (2) a metal orifice plate or nozzle member in which theorifices are formed in the required pattern; and (3) a silicon substratecontaining a series of thin film resistors, one resistor pervaporization chamber.

[0005] To print a single dot of ink, an electrical current from anexternal power supply is passed through a selected thin film resistor.The resistor is then heated, in turn superheating a thin layer of theadjacent ink within a vaporization chamber, causing explosivevaporization, and, consequently, causing a droplet of ink to be ejectedthrough an associated orifice onto the paper.

[0006] In U.S. application Ser. No. 07/862,668, filed Apr. 2, 1992,entitled “Integrated Nozzle Member and TAB Circuit for InkjetPrinthead,” a novel nozzle member for an inkjet print cartridge andmethod of forming the nozzle member are disclosed. This integratednozzle and circuit design is superior to the orifice plates for inkjetprintheads formed of nickel and fabricated by lithographicelectroforming processes. A barrier layer includes vaporizationchambers, surrounding each orifice, and ink flow channels which providefluid communication between a ink reservoir and the vaporizationchambers. A flexible electrical conductor having conductive tracesformed thereon has formed in it nozzles or orifices by Excimer laserablation. By providing the orifices in the electrical conductor itself,the shortcomings of conventional electroformed orifice plates areovercome. The resulting printhead assembly having orifices andconductive traces may then have mounted thereon a substrate containingink ejection elements associated with each of the orifices. The leads atthe end of the conductive traces formed on the back surface of theprinthead assembly are then connected to the electrodes on the substrateand provide energization signals for the ink ejection elements.

[0007] An existing solution for bonding the conductive traces formed onthe back surface of the printhead assembly to the electrodes on thesubstrate includes the solderless electrical connection of two contactelements by using a laser light beam attached to a fiber optic systemwhich directs the light to the spot to be bonded. The method results insolderless gold to gold compression bonding of conductive leadscontained in a polymer flex circuit tape, such as a polyamide, withoutdamaging the tape. A strong solderless gold to gold bond can be formedbetween the gold plated copper lead on the flex circuit tape and a goldplated pad on a semiconductor chip. As with all bonding procedures amethod for determining bad or low strength bonds is required. Damagedbonds or low strength bonds are usually detected by a sampling plan.This method utilizes shear tests to measure low bond strength. This is adestructive test and must use a small number of samples. Another methodto detect a bad bond utilizes an IR feedback to report any damage to thebond as a result of burning. This is a non-destructive method, however,it is only capable of detecting a burned bond. A low strength bond maynot be detected by this method.

[0008] Accordingly, it would be advantageous to have a process topredict and eliminate bad or low strength bonds caused by fiber damageduring laser TAB bonding process without destructive testing.

SUMMARY OF THE INVENTION

[0009] The present invention provides a method for predicting andavoiding bad bonds or connections when performing solderless electricalconnection of two contact elements by using a laser light beam attachedto a optical fiber system which directs the light to the spot to bebonded. The method provides for the detection of damaged optical fibersbefore bad bonds or connections occur. The method of the presentinvention performs optical fiber push laser bonding operations onelectric conductor leads includes providing an optical fiber push laserbonding system having an optical fiber for directing a laser beam,positioning first and second electrical leads in a bonding position,holding the first and second electrical leads in contact at a bondsurface with an optical fiber, bonding the first and second electricalleads at the bond surface by directing the laser beam through theoptical fiber, repeating said positioning, holding and bonding steps fora plurality of bonds, interrupting the aforesaid laser bondingoperations in order to examine the condition of the fiber; wherein thefollowing procedures occur during said interrupting directing the laserbeam through the optical fiber, capturing the spatial energydistribution of the laser beam exiting the optical fiber and analyzingthe spatial energy distribution of the laser beam to determine conditionof the optical fiber in order to determine the need for correctiveaction.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a perspective view of an inkjet print cartridgeaccording to one embodiment of the present invention.

[0011]FIG. 2 is a perspective view of the front surface of theelectrical conductor removed from the print cartridge of FIG. 1.

[0012]FIG. 3 is a highly simplified perspective view of the back surfaceof a printhead assembly having an electrical conductor with a siliconsubstrate mounted thereon and the conductive leads of electricalconductor attached to the substrate.

[0013]FIG. 4 is a side elevational view in cross-section taken alongline A-A in FIG. 5 illustrating the attachment of conductive leads toelectrodes on the silicon substrate.

[0014]FIG. 5 is a schematic diagram for a fiber push connect lasersystem as used with the present invention.

[0015]FIG. 6 shows in detail the electrical conductor, the contact bondpoint, the electrical conductor lead and substrate electrode.

[0016]FIG. 7 is a schematic diagram for a fiber push connect lasersystem of the present invention.

[0017]FIG. 8 illustrates the spatial energy spectrum of a clean fiber.

[0018]FIG. 9 illustrates the spatial energy spectrum of a damaged fiber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] While the present invention will be described, for purposes ofillustration only, in conjunction with the bonding of conductive leadson a electrical conductor to the silicon substrate of an inkjetprinthead, the present method for predicting and avoiding bad bonds orconnections when performing the solderless electrical connection of twocontact elements by using a laser light beam attached to a fiber pushoptic system is applicable to bonding other types of electrical membersto each other.

[0020] Referring to FIG. 1, reference numeral 10 generally indicates aninkjet print cartridge incorporating a printhead according to oneembodiment of the present invention simplified for illustrativepurposes. The inkjet print cartridge 10 includes an ink reservoir 12 anda printhead assembly 14, where the printhead assembly 14 includes aflexible polymer electrical conductor 18, a nozzle member 16 comprisingtwo parallel columns of offset holes or orifices 17. The orifices 17 maybe formed in the electrical conductor 18, for example, by laserablation.

[0021] A back surface of the electrical conductor 18 includes conductivetraces 36 formed thereon using a conventional photolithographic etchingand/or plating process. These conductive traces 36 are terminated bycontact pads 20 on the front surface of the electrical conductor 18. Theprint cartridge 10 is designed to be installed in a printer so that thecontact pads 20 contact electrodes on the printer carriage that provideexternally generated energization signals to the contact pads 20.Bonding occurs in bonding regions 22, 24 where the conductive traces 36of electrical conductor 18 are bonded to electrodes 40 (shown in FIG. 4)on a silicon substrate 28.

[0022] In the print cartridge 10 of FIG. 1, the electrical conductor 18is bent over the back edge of the print cartridge “snout” and extends upthe back wall and front wall of the snout. The contact pads 20 locatedon the electrical conductor 18 is secured to the front wall of the snoutand the conductive traces 36 are routed over the bend and are connectedto the substrate electrodes in the bonding region 22, 24 of theelectrical conductor 18.

[0023]FIG. 2 shows a top plan view of the printhead assembly 14 of FIG.1 removed from the print cartridge 10. The electrical conductor 18 hasaffixed to the back a silicon substrate 28 containing a plurality ofindividually energizable thin film resistors. Each resistor is locatedgenerally behind a single orifice 17 and acts as an ohmic heater whenselectively energized by one or more pulses applied sequentially orsimultaneously to one or more of the contact pads 20. The electricalconductor 18 shown in FIG. 2 has enough conductive traces 36 to controlapproximately 300 resistors on the substrate 28. The conductive traces36 may be of any size, test, and pattern, and the various figures aredesigned to simplify and clearly show the features of the invention. Therelative dimensions of the various features have been greatly adjustedfor the sake of clarity.

[0024]FIG. 3 shows a highly simplified schematic of the back surface ofthe printhead assembly 14 of FIG. 2 showing the silicon die or substrate28 mounted to the back of the electrical conductor 18 and also showingone edge of the barrier layer 30 formed on the substrate 28 containingink channels and vaporization chambers. Shown along the edge of thebarrier layer 30 are the entrances to the ink channels 32 which receiveink from the ink reservoir 12. The conductive traces 36 formed on theback of the electrical conductor 18 terminate in contact pads 20 and inleads 34 for bonding to the substrate electrodes 40. The bonding areas22 and 24 locate where the leads 34 of the conductive traces 36 and thesubstrate electrodes 40 are bonded.

[0025]FIG. 4 shows a side view cross-section taken along line A-A inFIG. 3 illustrating the connection of the ends of the leads 34 ofconductive traces 36 to the electrodes 40 formed on the substrate 28. Aportion 42 of the barrier layer 30 is used to insulate the ends of theconductive traces 36 from the substrate 28. Also shown is a side view ofthe electrical conductor 18, the barrier layer 30, the bonding areas 22and 24, and the entrances of the various ink channels 32. Droplets ofirk 46 are ejected from orifice holes associated with each of the inkchannels 32. Electrical conductor 18 may be Kapton™, Upilex™, or similartype polymer electrical conductor. Some such films may comprise teflon,polyamide, polymethylmethacrylate, polycarbonate, polyester, polyamidepolyethylene-terephthalate or mixtures thereof.

[0026] A schematic for a Fiber Push Connect (“FPC”) laser bonding system200 is illustrated in FIG. 5. This system consists of an Nd YAG or Diodelaser 202, equipped with a glass optical fiber 204. The system guidesthe laser beam to the attach point or bonding region 22, 24 via theoptical glass fiber 204. An optimum thermal coupling is achieved bypressing the lead 34 and substrate electrode 40 together by means of theoptical fiber 204 which creates a zero contact gap between the lead 34and substrate electrode 40 and thus improved thermal efficiency. Thisforcing of the electrical leads by the optical fiber 204 eventuallyleads to damage of the optical fiber as discussed below. FIG. 6 shows across-sectional view of the electrical conductor 18, the bonding regions22, 24, leads 34 on conductive traces 36 and substrate electrodes 40.

[0027] A feedback temperature loop is achieved by means of an infrareddetector 212 through the glass optical fiber. The temperature orabsorption behavior response of the IR-radiation reflected by thecontact elements 34, 40 at the bonding regions 22, 24 is gathered. Theoutgoing laser beam 220 from the laser source 202 goes through ahalf-transmission mirror or beam splitter 214 and through a focussinglens 216 into the glass optical fiber 204. The reflected light 218 fromthe optical fiber shown with dashed lines is reflected by the halfmirror 21 and arrives via focussing lens 222 at an IR detector 212 thatis connected to a PC Controller 224. The graph shown on the monitor 226of PC controller 224 is meant to show that the PC Controller 224 canstore definite expected plots for the temperature variation of thebonding process with which the actual temperature variation can becompared. The PC Controller 224 is connected with the laser source 202so that the laser parameters can be controlled if necessary.

[0028] The reproducibility of a FPC laser bond depends both on a highdegree of thermal coupling between the two connectors 34, 40 and highabsorption of the laser energy by conductive leads 34, 40. To optimizethe bonding process, minimum absorption is desired in the Kaptonelectrical conductor and maximum absorption is desired in the electricalconductor 18 metal layer. Metals with higher absorption rate willtransform a higher share of the laser energy into heat. This will resultin a shorter attach process which in turn will result in a higherquality bond.

[0029] The laser utilized is a YAG laser with a wavelength of 1064 nm.The YAG laser beam passes through the layer of polyamide without anyabsorption. A layer is required to provide a material which absorbs thelaser energy. Chromium and molybdenum have the highest absorptioncharacteristics at this wavelength. Chromium is often used as the seedmetal because most electrical conductor manufacturers are already usingchromium extensively to provide an adhesion layer between the coppertrace and Kapton polyamide in electrical conductor manufacturingprocesses.

[0030] The laser beam creates a localized heated zone in the chromiumcausing the metals (or solder material), to melt and create a bondbetween two joining electrical members without increasing thetemperature of the Kapton electrical conductor. However, any gap betweenthe two mating metal parts will cause over heating of the metal surfaceexposed to the laser beam. The optical fiber is used to push the twomating metal parts together to avoid any gap.

[0031] Further details regarding fiber push laser bonding are describedin U.S. patent application Ser. No. 08/843,492, filed Apr. 16, 1997,entitled, “Solderless Connection of Electrical Contacts UtilizingCombination Laser and Fiber Optic Push Connect System” and U.S. patentapplication Ser. No. 08\705,928, filed Aug. 30, 1996, entitled “LaserWelded inkjet Printhead Assembly Utilizing a Combination Laser and FiberOptic Push Connect System.” The foregoing commonly assigned U.S. patentapplications are herein incorporated by reference.

[0032] In the windowless electrical conductor bonding process, a fiberis utilized to transfer the laser energy from laser to the bond site.The quality of the bond joint is adversely affected by damage to thefiber tip. Due to repeated impact of the fiber tip, the fiber graduallygets damaged. This damage to the fiber will cause a higher portion ofthe laser energy to be lost, and not be absorbed by the electricalconductor lead. Therefore, bonds are made with lower laser power, andless loss of strength. Since no burning has occurred, laser IR detectordoes not detect any laser energy variation which in turn means no badbond is detected.

[0033] Referring to FIG. 7, the present invention uses a far fieldpattern optical lens 300, a CCD camera 302 and imaging software tocapture a spatial view of the laser power spectral output at the end offiber 204. In FIG. 7 no electrical leads are in position when the laserbeam analysis is taking place. The far field optical lens 300 isspecialized for measuring the far field pattern (FFP) of the beam of anoptical fiber in real time. The output pattern shows the two dimensionalangular intensity distribution of the light beam. You can measurevarious parameters of FFP using it with a video camera and an imageanalyzing processor. The far field pattern optical lens 300 consists ofa F-θ lens, a field lens and a relay lens. The F-θ lens is key devicefor the FFP Optics. A suitable far field pattern optical lens isCoherent's Model A3267-05. The output of the FFP lens is captured by theCCD camera 302.

[0034] Charge-coupled-device (CCD) cameras are solid-state devices withmany useful characteristics for doing laser-beam diagnostics. The CCDcamera, in conjunction with digitizing and processing electronics,primarily have two features. First, they give a picture of the beamprofile so that the user can effectively see what the profile lookslike. The fast response in both two- and three-dimensional modesprovides an insightful, intuitive perspective on the beamcharacteristics. Second, digitizing electronics can extract detailedquantitative measurements on beam characteristics, which allows the userto precisely determine the properties of the laser beam and to makeadjustments and improvements in its performance. The quantitative andvisual feedback complement each other by simultaneously enabling preciseoperation. In addition, the quantitative data provide a permanent recordof the setup and results. An example of a suitable CCD camera isCoherent's Model COHU 48.

[0035] The illuminance distribution pattern obtained by the CCD camerais equivalent angular intensity distribution of light source. In otherwords, the image obtained by FFP optics is the radiation angle patternprojected on hemispherical screen from luminous point of view.

[0036] The output of the CCD camera is transmitted to PC via line 304which is connected to a PC Controller 224. The graph shown on themonitor 226 of PC controller 224 is meant to show that the PC Controller224 and monitor 226 can store and display a spatial spectrum of thelaser beam at the tip 205 expected plots for the temperature variationof the bonding process with which the actual temperature variation canbe compared. The PC Controller 224 is connected with the laser source202 so that the laser parameters can be controlled if necessary. The PCcontroller 224 holds and executes the video laser beam analyzingsoftware.

[0037] The beam analyzing software is a standard laser beam analysissoftware that allows for capturing and displaying peak intensity andposition, peak power density, relative power and energy, area uniformityand beam diameter. An example of suitable beam analyzing software isCoherent/Auburn Group's BeamView Analyzer PC software. The foregoingsetup enables a one to view the spatial spectrum of the laser beam as itexits the fiber.

[0038] The analysis and method of the present invention at intermittentintervals by interrupting the bonding process and performing theanalysis of the laser beam at the tip 205 of the optical fiber 204. Whenthe analysis determines that the optical fiber is defective, the fibercan either be replaced or repaired if that is possible. It is importantto stress that it is spatial energy spectrum distribution across thefiber that is used in the method of the present invention. The absoluteenergy level is not important for the analysis of the present invention.

[0039]FIG. 8 illustrates the spatial energy spectrum of a clean fiber.The laser energy strength color map is illustrated to the left of thepicture. As it can be observed from FIG. 7, the laser energy is welldistributed spatially over the fiber and no “hot spots” are observed.

[0040]FIG. 9 illustrates the spatial energy spectrum of a damaged fiber.In this case several “hot spots” indicated by purple and red may clearlybe observed. “Hot spots” result in localized heating of the polyamideand bonds. This results in local burning and lack of a uniformly strongbond. This process of the present invention may be used to evaluate theoptical fiber and thus predict and avoid bad bonds and bond jointstrength without any destructive testing by evaluating the spatial ofthe fiber. The process of the present invention also reduces thequantity of parts that have to scrapped.

[0041] The foregoing has described the principles, preferred embodimentsand modes of operation of the present invention. However, the inventionshould not be construed as being limited to the particular embodimentsdiscussed. Thus, the above-described embodiments should be regarded asillustrative rather than restrictive, and it should be appreciated thatvariations may be made in those embodiments by workers skilled in theart without departing from the scope of the present invention as definedby the following claims.

What is claimed is:
 1. A method of performing optical fiber push laserbonding operations on electric conductor leads, comprising: providing anoptical fiber push laser bonding system having an optical fiber fordirecting a laser beam; positioning first and second electrical leads ina bonding position; holding the first and second electrical leads incontact at a bond surface with an optical fiber; bonding the first andsecond electrical leads at the bond surface by directing the laser beamthrough the optical fiber; repeating said positioning, holding andbonding steps for a plurality of bonds; interrupting the aforesaid laserbonding operations in order to examine the condition of the fiber;wherein the following procedures occur during said interrupting:directing the laser beam through the optical fiber; capturing thespatial energy distribution of the laser beam exiting the optical fiber;and analyzing the spatial energy distribution of the laser beam todetermine condition of the optical fiber in order to determine the needfor corrective action.